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New Scientist Live

Saturn’s moons could reassemble after a cosmic smash-up

Debris from moon collisions sticks itself back together within decades

CASSINI-HUYGENS/NASA/ESA/ISA

By Jeff Hecht

Saturn’s mid-sized moons are like the monsters in late-night horror movies. Smash them into tiny pieces, and they glue themselves back together as new versions of the old moons. This new finding contradicts a theory that Saturn’s rings were caused by moons colliding.

All four giant planets in our solar system have rings, but Saturn’s are by far the brightest and most massive. It’s not yet clear where the material needed to form rings comes from.

Last year, a study of Saturn’s moons pointed to a collision as the cause of the gas giant’s rings. Matija Cuk of the SETI Institute and colleagues found that the orbits of Tethys and Dione hadn’t changed much since the solar system formed more than 4 billion years ago.

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They suggested that resonant interactions of an earlier generation of moons caused a catastrophic collision just 100 million years ago that resulted in the debris that makes up Saturn’s rings.

Their theory was that largest pieces of this debris then formed Tethys, Dione and Rhea, while fine particles spread out to form rings. These mid-sized moons orbit Saturn in the diffuse area beyond its rings, and as time goes on, they hoover up everything in their path, so particles that spread inward formed the present rings inside the so-called Roche limit, where tidal forces break up large objects like moons.

To investigate if a collision could have caused the rings, Sébastien Charnoz and Ryuki Hyodo of the Paris Institute of Earth Physics in France modelled the event.

They found that if big chunks of the moons survived this smash-up, the debris would form a single new moon so fast that particles would not drift inward to form rings.

On the other hand, if the impact completely shattered both original moons, it could form two or more new moons, and “the particles would stop spreading within a few tens of years, giving them no time to spread enough to reach the Roche limit” and form the rings, says Charnoz.

“The process is so efficient that 20 to 30 generations of moons could have formed,” he says.

“This is an important result,” says Larry Esposito of the University of Colorado in Boulder, a specialist in planetary rings. “Although some large moons may repeatedly reform, Saturn’s rings cannot be produced by this scenario.”